1. Field of the Invention
The present invention generally relates to a transporting apparatus, and more particularly to a transporting apparatus using a linear motor which directly converts an electric energy into a linear mechanical energy.
2. Prior Art
Conventionally, a linear motor as shown in FIGS. 1A and 1B is known as a linear motor having a construction of a linearly developed permanent magnet type synchronous motor. In FIGS. 1A and 1B, a primary unit 1 consists of a primary core 3 having a layer construction which is constructed by a plurality of electric iron plates. Each electric iron plate 2 has teeth 2a and channels 2b. In addition, each of three coils 4u, 4v and 4w is winding at each channel 2b. When three-phase alternating currents are supplied to the coils 4u, 4v and 4w, the primary unit 1 generates a progressive magnetic field which progresses in a direction A or B. A half wavelength (or a length of half cycle) of this progressive magnetic field is called a pole pitch p in particular.
Meanwhile, 6 designates a secondary unit which faces the primary unit 1 with a gap G. This secondary unit 6 is constructed so that a plurality of permanent magnets 8 can be disposed on a long base plate 7. The length of each permanent magnet 8 is set equal to the pole pitch p. These permanent magnets 8 are disposed so that the north (N) pole and the south (S) pole of the permanent magnets 8 can face the primary unit 1 in turn. In addition, a supporting mechanism (not shown) keeps the gap G between the primary unit 1 and the secondary unit 6 constant. Due to this supporting mechanism, the primary unit 1 can be freely moved in the direction A or B. For example, when the primary unit 1 is fixed at a certain position and the three-phase alternating currents are supplied to the coils 4u, 4v and 4w, the secondary unit 6 can move in the direction A or B in synchronism with a moving speed of the progressive magnetic field generated by the primary unit 1 based on an operating principle of the permanent magnet type synchronous motor. Similarly, when the secondary unit 6 is fixed instead of the primary unit 1, the primary unit 1 can move in the direction A or B.
In order to obtain an efficient thrust power of the linear motor having the above-mentioned construction, it is required to increase a magnetic flux density generated by the permanent magnets 8 of the secondary unit 6. However, the permanent magnets 8 are arranged adjacently in the conventional linear motor. Hence, the conventional linear motor is disadvantageous in that a rare earth magnet having a high price must be used as each permanent magnet 8 in order to increase the magnetic flux density. In addition, magnetic pole planes of the permanent magnets 8 face the primary unit 1. Hence, when rush currents flow into the coils 4u, 4v and 4w, the magnetic fluxes generated from each tooth 2a of the primary core 3 must flow through the permanent magnets 8. Thus, the conventional linear motor is disadvantageous in that the permanent magnets 8 must be demagnetized.
Meanwhile, semiconductors are produced in a so-called "clean room" within a factory. Such clean room has a highly dustless condition. In order to produce the semiconductors, there are many processes for transporting wafers. In addition, transporting lines by which the wafers are transported includes vertical transporting lines so that the transporting lines are formed three-dimensionally. As an transporting apparatus used for the vertical transporting lines in particular, the following apparatuses are used conventionally.
(1) It is possible to employ a lifter which is driven by chains having a power source such as a hydraulic pressure source, air pressure source or a motor.
(2) It is possible to employ an electric truck which can be transported up and down by use of a motive power of a motor transferred thereto via rack and pinion.
(3) It is possible to employ a ground primary method of a linear motor as shown in FIG. 2. In FIG. 2, a primary unit 11 of a linear motor is put on a rail 12, and a secondary unit 14 is mounted to a transporting truck 13. This transporting truck 13 can be driven by the thrust power of the aboveconstructed linear motor by use of wheels 15a to 15d.
(4) It is possible to employ a ground secondary method of a linear motor as shown in FIG. 3. In FIG. 3, the primary unit 11 of the linear motor is mounted to the transporting truck 13, and the secondary unit 14 is put on the rail 12. In addition, a trolley rail 17 is mounted to the rail 12. According to such method, the three-phase alternating currents from the trolley rail 17 are supplied to the primary unit 11 via current collector brushes 16a to 16c, whereby the linear motor will be driven.
In the case where the lifter is employed as the transporting apparatus which is used in the highly dustless clean room within the semiconductor producing factory as described before, a contact portion must be formed in a mechanical portion such as the chain. Similarly, such contact portion is formed between the rack and the pinion of the electric truck. In addition, such contact portion is formed between the rail 12 and the wheels 15a to 15d shown in FIG. 2. In FIG. 3, such contact portion is formed between the rail 12 and the wheels 15a to 15d and between the trolley rail 17 and the current collector brushes 16a to 16c. Therefore, when the transporting truck having such contact portion is transported, the dust must be produced from the contact portion. In order to prevent such dust from being produced, a cover must be provided for the transporting apparatus. In addition, workers must take great care of an air flow in the clean room, and the workers must take great care of the dust so that the dust may not produce a bad effect on the wafer of the semiconductor.